1. Field of the Invention
The present invention relates to a link mechanism for a chair that is used in a chair, and relates to a chair.
2. Description of the Related Art
As chairs that are used in offices and the like, there have conventionally been proposed chairs in which, when a user rests against the backrest, i.e., the back surface portion, the seat surface portion operates interlockingly with the back surface portion (see, for example, Japanese Patent Application National Publication No. 2000-505677, Japanese Patent No. 4037438, and the “Modus:Function” section of the homepage of Wilkhahn at http://www.wilkhahn.co.jp/products/working/modus/function.html).
FIG. 14 is a drawing showing the structure of a conventional chair.
In FIG. 14, 113 is a base portion of a conventional chair. The base portion has supports, casters, and the like that are not illustrated, and is placed on a floor surface, and supports the mass of the entire chair and a user seated on the chair.
A seat surface portion 114, on which a user sits, is rotatably mounted to the upper end of the base portion 113 via a joint portion. Further, a first link 112 that supports a back surface portion 115 is rotatably mounted via a joint portion to an intermediate portion of the base portion 113.
The seat surface portion 114 and the first link 112 are connected by a second link 111 that is rotatably mounted to the both via joint portions.
When a user who is seated on the seat rests against the back surface portion 115, the first link 112 that supports the back surface portion 115 rotates around the joint portion with respect to the base portion 113. Further, because the seat surface portion 114 is connected to the first link 112 by the second link 111, interlockingly with the first link 112, the seat surface portion 114 rotates around the joint portion with respect to the base portion 113.
However, in the above-described conventional chair, the seat surface portion 114 does not operate unless force is applied to the back surface portion 115, and therefore, the user cannot always assume an optimal seated posture.
Namely, the seat surface portion 114 does not operate if the user who is seated on the seat surface portion 114 does not tilt his/her back more than the angle of inclination of the back surface portion 115 with respect to the seat surface portion 114 in the initial state.
Accordingly, in a case in which the user who is seated on the seat surface portion 114 does not rest against the back surface portion 115, such as, for example, a case in which the user is working while facing his/her desk, the angle of the seat surface portion 114 does not change. Therefore, the user cannot always assume an optimal seated posture.
For example, when the user is seated with a forward-leaning posture so as to face a desk, the user is not resting against the back surface portion 115, and thus, the back surface portion 115 does not fit the lumbar region, and the posture of the person who is seated deteriorates.
Thus, a construction that is such that the user of a chair can always assume an optimal seated posture is desired.